Twenty years ago, a group of clients built the first passive house in Darmstadt. Since then, energy regulations in architecture have gradually become stricter, but the energy use limit for passive houses has remained at 15 kWh of heating energy per square metre and year. Up to this level the temperature in buildings can be regulated solely by means of incoming fresh air, without the need for an additional heating system. If a building uses more thermal energy, this method of regulating temperature is no longer possible without reductions in comfort.